The effects of intrafraction motion on dose heterogeneity

Abstract

Intrafraction motion has long been suspected of causing inaccuracies in the resultant dose delivered to the patient. This study attempts to determine how breathing motion affects intact-breast cancer patients for three different treatment techniques commonly used in the clinic: physical wedge compensators (PWs), enhanced dynamic wedges (EDWs), and step-and-shoot intensity modulated radiation therapy (ssIMRT). Some of the effects of intrafraction motion can be eliminated with Gating Therapy. In this study, we have also investigated the efficiency of a Real-Time Position Management Gating System. In order to mimic patient breathing, equipment has been designed to simulate respiratory motion to a first order approximation. A breast phantom has also been designed to represent patient tissue and shape. Film was used as a dosimeter and static dosimetry data were used as a control for comparison. Three velocities of the breast phantom were studied, and Gating Therapy was introduced for each data set. Dose area histograms were calculated for a breast and lung planning target area (PTA), and Normalized Agreement Test (NAT) Indexes were calculated in reference to the static case. Our study shows that the results are dependent on the respiratory rate and the wedge angle and that deviation from the static case is highest if the collimator speed is of the same magnitude as the speed of the target. Generally, there is a large overdosage to the lung PTA and a large underdosage to the breast PTA. However, with the implementation of Gating Therapy, these dose discrepancies are dramatically reduced. The areas of high and low dose within the treatment field observed in this study can be related to other treatment sites affected by respiratory motion. Based on these results, we highly recommend using Gating Therapy for all treatments that may be affected by intrafraction organ motion.